1. Field of the Invention
The field of the invention is that of spacecraft and more particularly that of artificial satellites or probes on trajectories or in orbits which are not geostationary. Because they are not geosynchronous, these spacecraft do not have stable and unvarying geometrical relationships to a celestial body such as the Earth, for example. Satellites in low Earth orbit are one example: seen from a fixed point on the Earth, these satellites "track" across the sky until they disappear beyond the horizon.
2. Description of the Prior Art
Satellites of this kind are currently deployed in low Earth orbit, for Earth observation missions in particular, in which application they have the advantage of being able to observe practically all of the Earth as they travel in orbit and as the Earth rotates upon itself and relative to the orbital plane. On the other hand, because the position of these satellites in the sky is not fixed when seen from the Earth, fixed ground stations are not always able to transmit to or receive from them.
This is a problem in that observation satellites can observe the Earth throughout their orbit but can transmit their observations for processing only when they are over a ground station. A first generation of observation satellites (for example ERS-1, ERS-2) has been deployed under these conditions and the result is global observation with gaps representing geographical areas where there are no ground stations for receiving data.
The conventional solution to this problem is to provide these satellites with onboard means for memorizing and temporarily storing observation data on board the satellite to be subsequently retrieved and transmitted to a ground station when the satellite passes over a ground station. In the systems currently deployed these systems use magnetic recording means, for example. This is the case with the SPOT satellites, for example.
This prior art solution is satisfactory provided that the data recording and retrieval means operate correctly. Because these means are critical to the mission of a low Earth orbit observation satellite it is usual for these satellite onboard means to be redundant, i.e. there are at least two complete recording equipments on board, the second being adapted to be activated from the ground in the event of failure of the first.
Failure of both equipments considerably handicaps the satellite in performing its mission as, no longer able to store information, it can only transmit in real time data from observations effected when the satellite is in direct line of sight with a ground station.
The prior art has previously put forward two solutions: either to increase the number of ground stations or to use telecommunication satellites in geostationary orbit as relays for transmitting data from the observation satellite to existing ground stations. Each solution has drawbacks associated with the cost of designing and installing the necessary infrastructure and of operating this as yet non-existent infrastructure.
An object of the invention is to improve upon this situation.